Electrolytic production of certain trichloropicolinic acids and/or 3,6-dichloropicolinic acid

ABSTRACT

Electrolytic reduction of tetrachloro-2-picolinic acid in basic aqueous solution, at an activated silver cathode, yields the 3,4,6- and 3,5,6-trichloro-2-picolinic acids, which in turn may be further reduced to 3,6-dichloropicolinic acid, a highly active herbicide.

BACKGROUND OF THE INVENTION

3,6-Dichloropicolinic acid is disclosed in U.S. Pat. No. 3,317,549, as ahighly active plant growth regulator which can be made by acidhydrolysis of 3,6-dichloro-2-(trichloromethyl)-pyridine.

In the approximately twelve years since the U.S. Pat. No. 3,317,549patent issued, 3,6-dichloropicolinic acid (3,6-D, henceforth) has becomeincreasingly of interest, particularly for control of weeds which cantolerate phenoxy-type herbicides--such as 2,4-D and MCPA. Thus, Canadathistle, Russian smartweed and wild buckwheat--for example--aresusceptible to 3,6-D. The latter compound also has shown good activityon rangeland brush species, such as velvet mesquite, catclaw acacia andwhitehorn. 3,6-D not only has a low toxicity to mammals, fish and birds,but is also relatively short lived in soil.

However, realization of the full potential of 3,6-D requires thedevelopment of a more efficient and economic method of synthesis thanthe preparation and hydrolysis of the corresponding trichloromethylcompound.

A substantially better method of preparing 3,6-D involves the reductionof 3,4,5,6-tetrachloropicolinic acid by successive reactions withhydrazine and an aqueous base, as described in U.S. Pat. No. 4,087,431.However, the efficiency of materials utilization in this process leavessomething to be desired and the process cost is relatively high to beborne by an economic herbicide.

Another method which could be considered is electrolytic reduction of3,4,5,6-tetrachloropicolinic acid. This of course would requirereplacement of both the 4 and 5 chlorines with hydrogen, underconditions such that hydrolysis (which is facile) and/or decarboxylationwould not occur to any substantial extent. It would also require thatover reduction, i.e., reduction of the 3,6-dichloropicolinic acidend-product--would not occur to a significant extend under theconditions required to effect the desired reduction.

U.S. Pat. No. 3,694,332 teaches that the 4-chlorine intetrachloro-2-cyanopyridine can be replaced by hydrogen if the lattercompound is co-dissolved with a neutral or acidic electrolyte (andwater, as necessary) in an organic solvent and subjected to electrolyticreduction at a mercury (or lead) cathode. The patent also teaches thatthe same method may be used to reduce pentachloropyridine to2,3,5,6-tetrachloropyridine (with coproduction of a small proportion ofan unidentified trichloropyridine). The use of strong bases aselectrolytes in the patented process is indicated as likely to result inhydrolysis reactions.

A co-pending application, Ser. No. 029,600, filed Apr. 13, 1979, in thename of D. Kyriacou as inventor, discloses that the electrolyticreduction of pentachloropyridine, co-dissolved with an aqueous base inan organic solvent and at a highly specific silver cathode, results inthe replacement of the 4-chlorine, and then the 6- (or 2-) chlorine,with hydrogen.

No more pertinent prior art then the foregoing U.S. Pat. No. 3,694,332patent is known of and the teachings of the patent and the aforesaidapplication fail to suggest to those knowledgeable in the prior art thattetrachloro-2-picolinic acid ("tet-acid") can be electrolyticallyreduced to the 3,6-dichloro acid (3,6-D) or to a trichloro-acid whichwill yield 3,6-D upon reduction. It may also be noted that attempts toelectrolytically reduce several other polychloro-pyridine carboxylicacids in aqueous base solutions have failed, even when a siver cathodeof the above mentioned type was employed. Similarly, attempts to soreduce a variety of chlorinated benzoic acids and phenols also wereunsuccessful.

3,5,6-Trichloropicolinic acid ("3,5,6-T") is also a known compound whichhas herbicidal activity but for which a really economic method ofsynthesis has not been published. 3,4,6-Trichloropicolinic acid("3,4,6-T") is a new compound (m.p. 128° C.) (not discovered by thepresent inventors). Due to the high reactivity of the 4-chlorine inpolychloropyridine compounds, it has been very difficult to devise apractical method of making 3,4,6-T. The only method known to havepreviously been found gives a very poor yield of the compound (and doesnot involve ═C--Cl reduction).

OBJECTS OF THE INVENTION

The primary object of the invention is to provide an efficient, economicmethod for the manufacture of 3,6-dichloropicolinic acid.

An additional object is to utilize 3,4,5,6-tetrachloropicolinic acid asa starting material for the preparation of 3,6-D or mixtures thereofwith 3,4,6- and 3,5,6-trichloropicolinic acid.

Another object is to accomplish the foregoing objects by electrolyticreduction.

It is a particular object to provide a method for electrolytic reductionof a tri- or tetrachloropicolinic acid without resort to organicsolvents, even though the acid has a low solubility in aqueous media.

A more specific object is to provide a method of electrolyticallyreducing tetrachloropicolinic acid, 3,4,6-trichloropicolinic acid or3,5,6-trichloropicolinic acid, wherein an undivided body of a solutionof the acid in an aqueous base serves as both catholyte and anolyte.

Yet another object is to provide a process in which, in a singleoperation, tetrachloro-2-picolinic acid can be converted to essentiallypure 3,6-D in yields of at least 90%.

A further object is to provide a practical method of producing3,5,6-trichloropicolinic acid, together with minor amounts of the3,4,6-isomer, from tetrachloro-2-picolinic acid.

Still other objects will be made apparent, by the followingspecifications and claims, to those skilled in the art.

SUMMARY OF THE INVENTION

It has now been discovered that a chlorine substituent in the 4- or 5-position of tetrachloro-, 3,4,6-trichloro or 3,5,6-trichloro-2-picolinicacid can be selectively replaced with a hydrogen by passing directelectrical current to a cathode from an anode through a basic aqueoussolution of said picolinic acid, if said cathode has a surface layer ofsilver microcrystals formed by the electrolytic reduction of colloidal,hydrous, silver oxide particles in the presence of an aqueous base.

It has been found that the mixed trichloropicolinic acids productobtainable by reduction of the tet-acid consists predominantly (up toabout 99 mole %) of one isomer--which is indicated by the evidence nowavailable to be 3,5,6-T. The balance of this product is believed to bethe 3,4,6-isomer.

It is not presently known how to modify the tet-acid reduction toincrease the proportion of 3,4,6-T in the tri-acid mixture. However,separation and accumulation of 3,4,6-T, as even a very minor by-productin ongoing 3,6-D manufacture, constitutes an improvement over the onlyother method known of for making 3,4,6-T.

The reactions involved in the reduction of tet-acid to 3,6-D may bedepicted as: ##STR1##

c. Anode reaction (for each --Cl replaced by an --H)

    2(OH.sup.-)→1/2O.sub.2 +H.sub.2 O+2e.sup.-

d. Overall reaction(s)

    I+2(OH.sup.-)→(III or IV)+Cl.sup.- +1/2O.sub.2 ↑+H.sub.2 O (1)

or

    I+3(OH.sup.-)→V+2Cl.sup.- +O.sub.2 ↑+H.sub.2 O (2)

If 3,4,6-T and/or 3,5,6-T is used as a starting material, the reactionsinvolved differ in that III or IV is formed directly by neutralizationof the corresponding acid, rather than by reduction of tet-acid anions.The overall reaction is then depictable as:

    (3,4,6-T or 3,5,6-T)+2(OH.sup.-)→V+Cl.sup.- +1/2O.sub.2 +H.sub.2 O. (3)

Thus, two chloride ions are formed and a total of three hydroxyl ionsconsumed for each tetrachloropicolinic acid molecule neutralized andreduced to the corresponding dichloropicolinate anion. If only one --Clin the starting material (tet-acid, 3,4,6-T or 3,5,6-T) is replaced byH, one chloride ion is formed and two hydroxyl ions are consumed.

More specifically, the present invention may be defined as:

an electrolytic process for the coproduction of oxygen andpolychloropicolinate anions of the structure ##STR2## wherein one X is Hand the other is H or Cl, said process comprising,

providing a solution in water of a hydroxyl ion source-material and apolychloropicolinic acid of the structure ##STR3## wherein both Z and Ware Cl, or one is Cl and the other is H,

immersing a cathode in a body of said solution and, while agitatng saidbody, passing an electric current therethrough from an anode to thecathode,

said body of solution having a temperature within the range of fromabout 5° to about 60° C., a pH of at least 13, and containing at least0.08 hydroxyl ions per chloride ion present therein,

said cathode comprising a shaped, electrical conductor in intimatecontact with a water and hydroxyl ion-containing, immobilized,metastable layer of aggregated silver microcrystals formed byelectrolytic reduction of colloidal, hydrous, silver oxide particles inthe presence of water and hydroxyl ions, the cathode having a potential,relative to a saturated calomel reference electrode, of from about -0.8to about -1.8 volts, and

said anode having a positive potential, relative to the cathode, suchthat the density of said current is from about 0.005 to about 0.085amperes per cm² of projected cathode surface,

thereby forming anions of said polychloropicolinic acid (A) at saidcathode and oxygen at said anode.

The use of a porous barrier between the catholyte and anolyte is notrequired and a single, stirred body of solution can function as bothcatholyte and anolyte.

In a less preferred embodiment of the invention,

a. the reduction mixture is a slurry of undissolved tetrachloropicolinicacid particles in a saturated solution of the acid in an aqueous alkalimetal hydroxide.

b. initially, all of said acid (as such or as a salt) to be charged, andall of the hydroxide to be charged, is present in the slurry,

c. initially, the number of moles of said acid and salt per equivalentof hydroxyl is within the range of from about 0.1 to about 0.2 and

d. the electrolysis is continued until at least 90% of the acid chargedto the reaction has been converted to the corresponding base salt of3,6-D.

In a more preferred method of practising the invention, the tri- ortetrachloropicolinic acid and/or a base is added during the course ofthe reduction.

Graphite is the preferred anode material in all variants of the presentinvention.

Also considered to be within the ambit of the present invention is anelectrolytic cell in which the cathode is as specified above and isimmersed in a catholyte comprising a solution of 3,4,6- or3,5,6-trichloropicolinic acid and/or 3,4,5,6-tetrachloropicolinic acidin an aqueous base.

DETAILED DESCRIPTION

It is critical to the successful practice of the present invention thatthe cathode used be an active silver cathode, as above defined and asdisclosed and claimed in said co-pending application, Ser. No. 029,600.Such cathodes and their preparation will now be described.

The essential physical-chemical nature of the metastable silver layerdepends on its being formed in the presence of and continuing to includehydrated hydroxyl ions.

In a first form of the cathode, the active silver layer is adhered to asurface of the shaped conductor. In a second form, the active layer is abody of silver powder constrained within a liquid-permeable bag orenvelope and the conductor either is surrounded (at least partially) bythe powder or constitutes the envelope which "surrounds" (contains) thepowder. In both forms, the conductor preferably consists of or is cladwith silver, i.e., is a silver monolith or is a composite conductorcomprising a conductive core sheathed with silver.

In the first form of the cathode, the microscopic topography of theactive surface layer can vary (according to details of substratecharacter and the silver oxide reduction procedure). For example, thesilver microcrystals may aggregate by packing closely together to form aplurality of mono- and polypartite "bumps" or dendrites protruding fromand contiguous with a surface portion of the conductor or, pack looselyto form discrete particles which cohere with each other and adhere tothe surface as a porous or "spongy" blanket which conforms to thesubstrate topography.

In the second form of the cathode, the silver particles initially formed(by reduction of the silver oxide particles) are in direct contact withthe conductor and function as part of the conductor for the formation ofthe next layer of particles, and so on. Similarly, contact between theconductor and the powder particles not in direct contact with it is byelectrical conduction through the intervening silver particles.

The silver oxide particles from which the active silver layer is derivedcan be formed in the immediate vicinity of the conductor, as byanodizing a silver conductor, or may be formed elsewhere and transportedto it in the catholyte--as by stirring.

If at least the portion of the conductor surface at which the silveroxide particles are to be reduced is defined by an outer layer ofsilver, the oxide may be generated in situ by anodization of the silver.Thus, monolithic silver or silver-clad composite conductors areparticularly convenient in affording electrodes which can be activated(or reactivated) in place in the cell to be used for carrying out areduction.

Accordingly, cathodes comprising silver or silver-clad conductors arepreferred. Such conductors may be of any configuration--such as ascreen, plate, rod, etc., which is suitable for their intended functionand which permits ready access of liquid to the silver surface at whichthe oxide reduction must proceed.

The presence of noble metals other than silver in the active surfacelayer has not been found beneficial and base metals (nickel and copper,most notably) definitely lower the activity of the cathode. Accordingly,it is highly desirable to minimize the content of metal ions other thansilver in the reduction medium around the conductor (or cathode) whenhigh 3,6-D yields are desired.

It then follows that, for maximum cathode activity, the conductor eithershould not be subjected to anodization or should consist of silver or amaterial which will not provide any substantial amount of ions (orreducible oxidation products) of other metals under the contemplatedanodization conditions. (Analytically pure silver is not required but isof course preferred.)

The outer (or substrate) silver layer in a composite conductorcomprising the same may be formed in any suitable manner, such as, forexample, by electroplating silver on an electroconductive "core" orunderbody. Since it is desirable for this layer to have a high surfacearea, the underbody surface on which the silver layer is formed does nothave to be--and preferably is not--smooth. Similarly, a method ofdepositing the outer layer which favors columnar or dendritic growth orotherwise results in a high surface area--but not in lateraldiscontinuities in the silver in immediate contact with theunderbody--is preferred.

In one variant of the cathode, the type of active overlayer whichconsists of protrusions contiguous with the silver substrate layer mayin turn serve as a once-removed substrate layer which can be induced, byperiodic polarity reversals, to cover itself with a conforming, porousblanket of cohered silver particles, thereby providing an outermostportion of the overlayer which has a still higher surface area structureof a character more like that of a "molecular sieve".

If the conductor is not to be subjected to anodization, it may consistof any otherwise inert, electroconductive material to which the silvermicrocrystals (as such or as particulate aggregates thereof) will adheresufficiently well not to be swept off by a stirred catholyte. Thisincludes nickel, stainless steel and--at least in principle--copper andgraphite. Similarly, if such a conductor is first clad with asubstantial thickness, of silver, it then becomes a composite conductorwhich may be activated by anodization.

As noted above, the colloidal silver oxide particles can be preformedand then introduced to the catholyte or can be formed in thecatholyte--optionally, in the layer of catholyte in immediate contactwith the silver base layer.

A convenient method of forming the oxide in a catholyte comprising waterand hydroxyl ions is simply to add a small amount of a water-solublesilver salt, such as silver nitrate, with sufficient agitation to keepthe resultant colloidal, hydrous, silver oxide particles well dispersed.Preferably, the salt is added as a dilute solution in water. Onceformed, the oxide particles contact and are reduced at thenegatively-charged cathode surface (the surface of the conductor) tobuild up the microcrystalline silver overlayer, i.e., to activate thecathode.

Formation of an active overlayer consisting of a non-adhered silverpowder is facilitated by using an envelope to effectively immobilize thesilver oxide precursor particles in close contact with each other and/orthe conductor; the reduction then more closely approximates anall-at-once production of the microcrystals and growth of earlier formednuclei (crystals) by accretion of later formed microcrystals isminimized.

The envelope may or may not be electroconductive, but must be readilypermeable to the aqueous phase of the catholyte. If it is conductive, itmay also function as the conductor. If it is not, a separate shapedconductive member (such as a wire or patch of metal screening, forexample) which can be inserted or embedded in the body of oxideparticles to be reduced, will be required. The envelope may beforaminous (as a fine meshed gauze or screen, for example) ormicroporous (as a TEFLON® or polyethylene diaphragm having a porosityand average pore size such as to permit a practical rate of transportthrough it of the aqueous tetrachloropicolinic acid salt solution).

The envelope should be so formed about the body of silver oxideparticles (to be reduced), as by flattening, crimping or stretching, asto exert some compression on it, thereby ensuring better contact betweenthe resultant silver powder particles.

If further activation (or reactivation) of the powder, by anodization,is not contemplated, the central conductor or conductive envelope canconsist of a metal such as nickel, stainless steel or even copper, towhich silver has less tendency to adhere, thereby further ensuring ahigh conversion of the silver oxide particles to unattached silverparticles.

In an alternative method of activation ("anodization") the silver whichmakes up the active overlayer is derived from the (silver) substratelayer itself. The unactivated electrode is dipped or immersed in acatholyte containing water and hydroxyl ions and is anodized (anodicallypolarized), thereby converting some of the silver in the base layersurface to colloidal silver oxide and roughening (corroding) the surfaceat the same time. The polarity of the electrode is then reversed and theoxide electrolytically converted to protrusions or particles ofmicrocrystalline silver (without re-smoothing the surface it is adheredto). Preferably, the polarity reversal is repeated several times atintervals of about 30 seconds. This same procedure can also be employedto reactivate a silver cathode which exhibits diminished activity.

Water plays an important, albeit not well understood, role in theformation and retention of the fine structure which is essential to theactivity (and selectivity) of the metastable silver overlayer. Bothwater and hydroxyl ions (hydrated hydroxide ions, at least) must beincluded in the overlayer when it is formed--and until it is no longerto be utilized in a cathode. Once formed, the microcrystallinesuperstructure will collapse somewhat and its activity will be at leastsubstantially diminished if its content of water is allowed to decreasebelow some minimum critical level. Whether or not this level correspondsto a monomolecular layer (over the entire fluid-accessible surface ofthe overlayer) is not known. Although the critical contents of water andbase can readily be determined for any specific, reproducible cathode,this is not necessary if the cathode is simply kept immersed in anappropriate basic, aqueous medium. Preferably, the latter medium issimply that in which the electrode was immersed when it was "activated",or is an aqueous base in which the tetrachloro acid to be reduced willbe dissolved.

The activation procedure is not adversely effected by the presence, withthe required water and base, of polychloropicolinate anions.Accordingly, activation most conveniently is carried out simply as thefirst step in the reduction for which the cathode is employed. This istrue for either of the two general activation methods discussed above,which will now be discussed in more detail.

In the activation method involving formation of the silver oxideparticles in the catholyte, it is not necessary to establish asubstantial silver content therein; silver contents of about 100 ppm(parts per million) are generally sufficient and contents substantiallyin excess of about 500 ppm are of no additional benefit and may actuallybe wasteful. The solution of the silver salt can be added to a preformedcatholyte mixture or to the aqueous base component thereof before thetetrachloropicolinic acid is introduced. The requisite degree ofagitation during the oxide deposition step is conveniently attained withmagnetic stirring.

The silver oxide particles to be reduced in forming an active silveroverlayer may be preformed in a separate container and then introducedto the catholyte--together with or separated from the aqueous medium inwhich they were formed. When the overlayer is to take the form of asilver powder, it is convenient to filter out the oxide particles and totransfer them (wet with aqueous base) to the envelope in which they willbe reduced. The envelope can be formed from a flat sheet of gauze orother suitable material on which the oxide particles have been placed,or may be a preformed, porous container (such as a porous fluorocarbonelastomer bag, for example) in which the particles are lightly packed.

In the activation method involving anodization of a silver substrate,the current density is usually controlled so that the potential at theelectrode surface rises, in a period of several minutes, from an intialvalue of, say, zero volts to a final value of at least +0.3 volts andpreferably about +0.6 volts. It is not necessary to add any silver tothe catholyte (or aqueous base) in this method.

It is known (Electrochemical Reactions, Charlot et al; pp. 298, 9;Elsevier Pub. Co., Amsterdam, N.Y., 1962) that the electrolyte oxidationof silver at progressively higher potentials in the presence of hydroxylions, results in the formation of not only Ag₂ O but also of higheroxides in which silver takes on nominal valences greater than one. Thecalculated positive (anode) potentials corresponding to formation of Ag₂O₂ and Ag₂ O₃, in about 13 N KOH, are, respectively, about +0.6 andabout +0.8 volts. A potential of about +0.6 volts is currently regardedas optimum for the in-situ preparation of silver oxide particles fromwhich the active silver layer is to be prepared; however, potentials ashigh as +0.8 volts are not ruled out.

Reduction of the oxide deposit requires negative polarization of thecathode in both methods of activation. In the first method discussedabove, the cathode potential is negative to start with and may rangefrom about -0.5 to about -2.0 volts; preferably it is from about -1.0 toabout -1.5 volts. In the second method, the polarization of the cathodeis gradually reversed. That is, the cathode potential is graduallyreduced from the value (about +0.3 to about +0.6 volts) attained in theoxidation step, to a value of about -0.5 volts or less (down to about-2.0 volts). In the first method, the current is relatively low in theearly stage of oxide reduction. Thereafter, the current will rise to anessentially steady value, assuming a reducible material(polychloropicolinate anions) is present in the catholyte. However, inthe second method, the current drops off from an initially higher levelto a minimum, at which point (potential about -0.5 volts) the oxidereduction is complete. If the potential is lowered further, the currentwill then increase--again assuming a reducible material is present--to avalue of about 1.5 amps (cathode potential about -1.0 to about -1.5volts).

Before the cathode is activated by either method, it preferably iscleaned, as by immersing it in aqueous hydrochloric acid (1:1 water andc.HCl) for about ten minutes. Similarly, when a cathode which has beenused as such for some time and is to be reactivated, it should first becleaned in the same manner, to essentially remove any detrimental metalswhich may have plated out on it.

As an exception to the foregoing teachings with regard to deleteriouseffects of certain metal ions which tend to plate out on the cathode,some deactivation of the silver micro-crystal layer at the cathodesurface is apparently desirable for the production of trichloropicolinicacids. The initial reduction of tet-acids to the trichloro-intermediatesproceeds more readily than the further reduction to 3,6-D. By using aless active cathode, the selectively of the reaction for the trichloro-compounds is increased.

Suitable anode materials for the practice of the present invention arethose which are inert, i.e., do not detrimentally react with any of thecatholyte components (or oxygen) to an intolerable extent. However,3,6-D yields (based on tet-acid charged) of 90% or better have beenattained only with anodes consisting essentially of graphite. This isapparently because decarboxylation ("Kolbe type" oxidation ofpolychloropyridine carboxylate anions) tends to occur at anodesconsisting of other materials.

The composition of suitable catholytes (reduction mixtures) for thepractice of the invention will now be discussed. The catholyte mustcomprise an aqueous phase containing both hydroxide ions and anions ofthe polychloropicolinic acid to be reduced. This phase may also includedissolved salts of 3,6-D and such by-products as may be formed in thecourse of the reduction.

Ordinarily, the hydroxide ions (and the required positive counter ions)will be provided by an alkali metal hydroxide. However, any otherwisesuitable source of hydroxyl and counter (cat) ions may be employed.Sodium hydroxide is highly preferred as the hydroxyl ion source material(base, herein). Commercially available "pure" (mercury cell) 50% aqueousNaOH has been found quite satisfactory. For the production of 3,6-D, atleast, it is highly preferred that the catholyte (the aqueous phase thatis) comprise less than about 20 ppm, total, of base-metal ions, butreagent grade caustic is generally not required.

The catholyte may also include a second phase which comprises 3,4,6-T,3,5,6-T or tet-acid and is dispersed or suspended in the aqueous phase.Preferably, neither this phase or the aqueous phase will contain anysubstantial proportion of other organic materials. However, dissolutionof the un-neutralized polychloro-acid(s) in an organic solvent which isessentially immiscible with the aqueous phase but permits transfer ofenough of the acid to the aqueous phase to keep the latter saturatedwith the acid salt to be reduced is considered feasible. This, ofcourse, is with the proviso that an intractable emulsion does not resultfrom inclusion of the solvent.

Similarly, the aqueous phase may include one or more dissolved organicsolvents, of such character and in such amounts as not to deleteriouslyeffect the cathode, the electrode reactions or product recovery to anintolerable extent. However, it is a distinct advantage of the presentinvention that conventional co-solvents--which are generally flammableand often toxic and/or prone to peroxide formation--are not required.

In the presently preferred mode of operation, the substrate acid (theacid to be reduced) is added incrementally to the catholyte as apowdered solid. Advantageously, this solid is pre-slurried with aportion of the catholyte (or with an aqueous base) before being added tothe cell. It has been found that undissolved tet-acid particles, whenwet with the aqueous phase, tend to aggregate as relatively large lumpswhich are then difficult to break up. Also, the particles tend to form a"foam" (which is unresponsive to defoaming agents) when contacted by thegases evolved in the reduction. Both of these difficulties are minimizedby slow addition of the acid, ideally at a rate about equal to the rateat which the dissolved acid salt is converted in the reduction.

For the preparation of 3,6-D in high yields, it is critically importantthat the pH of the aqueous phase of the catholyte be kept at a level ofabout 13 or higher throughout the reaction. It is also essential in thisregard that the number ratio of hydroxyl to chloride ions therein not beallowed to fall substantially below 0.6 (OH⁻ /Cl⁻ weight ratio 0.3).Otherwise, appreciable amounts of chloride oxidation products(hypochlorite, for example) and decarboxylated chloropyridines may format the anode.

The foregoing pH and OH⁻ /Cl⁻ conditions are considered similarlydesirable when the process of the invention is used to make 3,4,6-T and3,5,6-T from the tet-acid.

Preferably the OH⁻ to Cl⁻ equivalent (or number) ratio is kept at orabove a level of 1.

Ordinarily, the lowest OH⁻ /Cl⁻ ratio at which reduction will be carriedout will occur just prior to the end of the reduction, i.e., when themost OH⁻ has been consumed and the most Cl⁻ produced. At any stage ofreaction, the minimum value to which the ratio can fall will be thatattained if the reduction is allowed to proceed until all of the tri-and/or tetrachloro-acids charged have been reduced to 3,6-D. Thisminimum value is equal to (a-3b-2c)÷(2b+c+d), where a is the gram ionsof OH⁻ charged, b is the gram moles of tet-acid, charged, c is the grammoles of 3,4,6-T and/or 3,5,6-T charged and d is the gram ions of Cl⁻charged (initially present). If c and d are nil, the foregoing quotientreduces to (a-3b)÷(2b). If b and d are nil, the expression insteadreduces to (a-2c)÷c.

The assumption that no side reactions occur is implicit in the way thequotient is defined and the value calculated for it is therefore atheoretical minimum which will seldom be realized. However, in mostinstances, the actual lowest OH⁻ /Cl⁻ ratio attained will not greatlydiffer from the theoretical minimum. The latter quantity thus may beused as a practical criterion of the relative amounts of base andsubstrate acid(s) employed in a given reduction.

Thus, when tet-acid is to be converted to 3,6-D in high yield, the grammoles of tet-acid which can be charged per g. ion of OH⁻ charged,without causing the OH⁻ to Cl⁻ ratio to drop substantially below 0.6, isfound by setting 0.6 equal to (1-3b)÷2b and solving for b; i.e.,b=˜0.24. Similarly, if 3,5,6-T is to be converted to 3,6-D and an OH⁻/Cl⁻ ratio of at least 0.6 maintained, 0.6=(1-2c)÷c and c=0.38; i.e.,not more than 0.38 gram moles of 3,5,6-T should be charged per gram ionof OH⁻ charged.

So far, reduction of tet-acid to 3,4,6-T, without co-formation of3,5,6-T and 3,6-D in substantial proportions, has not been achieved. Thegeneral expression for the theoretical final OH⁻ /Cl⁻ ratio in this caseis (a-b(3x+2y+z))÷(b(2x+y)+d), where a, b and d are as above defined, xand y are, respectively, the mole fractions of the tet-acid converted to3,6-D, and to "tri-acids" and z is the mole fraction of tet-acidunconverted. Thus, in a typical reaction, 1 gram mole of tet-acidproduces a mixture for which z=0.1, x=0.5 and y=0.4. If a nil originalCl⁻ content and a theoretical final OH⁻ /Cl⁻ ratio of 0.8 are assumed,then a, the amount of OH⁻ required to be charged, is found from theequality, 0.8=(a-1(1.5+0.8+0.1))÷(1(1+0.4))=(a-2.4)÷1.4, to be at least3.52 gram ions (or 3.52 gram moles of an alkali metal hydroxide, forexample), i.e., the mole ratio of tet-acid to the hydroxide should notexceed 1/3.52 or 0.284).

It is apparent from Example 8c, herein, that even at a theoreticalminimum (or final) OH⁻ /Cl⁻ ratio as low as 0.08, tet-acid can beconverted to 3,6-D in about 77% yield. At the other extreme, if nochloride ions are initially present, the OH⁻ /Cl⁻ ratio at onset of thereduction is infinite; thus, there is no upper limit to this ratio.

When the hydroxyl ion source-material is NaOH (and tet-acid is the onlypolychloropicolinic acid substrate charged), the overall weight ratio oftet-acid to NaOH charged should be within the range of from about 0.5 toabout 2.1, but preferably is from about 0.65 to about 1.3. Thecorresponding mole ratio ranges are from about 0.075 to about 0.32 and(preferably) from about 0.1 to about 0.2; the latter ranges apply toalkali metal hydroxides in general.

The concentration of the hydroxyl-ion source material in the catholytecan range from that required as a minimum for a pH of 13 to that atwhich the solubility of the tet-acid salt of the base employed becomesimpractically low. In the case of sodium hydroxide, the latter range isfrom about 0.4 wt. % to about 15 wt. %. The preferred range for NaOH isfrom about 5 to about 7 wt. % (about 2.1 to about 3 wt. % OH⁻). In termsof moles, the latter ranges, respectively, are from about 0.1 to about3.75 and (preferably) from about 1.25 to about 1.75 gram moles of NaOHper 1000 grams of catholyte (H₂ O, NaOH, substrate acid). Approximatelythe same ranges are considered suitable for other alkali metalhydroxides.

The relative amount of 3,4,6-T, 3,5,6-T and/or tet-acid which can bepresent in the catholyte as undissolved materials should not exceedabout 12 weight percent of the catholyte; the slurry is undesirablyviscous at higher levels. The tri- and tet-acid salts (such as thesodium salts, for example) are soluble in strong aqueous bases (10%NaOH, for example) only to the extent of a few percent by weight, so thetotal content of unconverted acids (dissolved and undissolved willgenerally not exceed about 15 wt. %. Of course, more tri- or tet-acidmay be introduced as the reaction proceeds, so long as this does notresult in an OH⁻ /Cl⁻ number ratio of less than about 0.08. Likewise,more hydroxide-ion source material may also be added, but the catholytemust retain sufficient fluidity and solvent ability (for the acid salt)to ensure stirrability and an adequate reaction rate.

When the entire amount of polychloro-acid to be charged is not initiallypresent in the catholyte, the balance may be added as the free acid oras a preformed salt of the base employed, together with as much extrabase as may be required to ensure that the OH⁻ /Cl⁻ ratio does not droptoo low.

It has been found that the 4-chlorine substituent in the tet-acid (or in3,4,6-T) tends to undergo base hydrolysis, even at room temperature.Consequently, a basic, aqueous solution of such a polychloropicolinicacid which is to be electrolyzed should either be freshly prepared orkept cold.

Suitable temperatures for the electrolysis generally range from about 5°to about 60° C. At temperatures above 50°, side reactions (such ashydrolysis) occur to a sufficient extent to seriously effect yields,complicate 3,6-D and O₂ recovery and present by-product disposalproblems, and at temperatures below 10° C., tet-acid solubility isundesirably low. The preferred temperature range is from about 20° toabout 40° C. and the most preferred range is from about 34° to about 36°C.

Suitable contact times, for tet-acid to 3,6-D conversions of 90% orbetter, range from about 12 hours (at about 20° C. ) to about 31/2 hours(at about 40° C.). Times in excess of 10 hours tend to result in someover-reduction and/or side reactions (particularly at temperatures above30° C.).

The rate of 3,4,6-T and 3,5,6-T reduction of course drops considerablyin the later stages of the reaction, so suitable contact times when thetri-acid(s) are employed as a pre-formed starting material are notgreatly shortened. However, if production of 3,4,6-T and 3,5,6-T (and3,6-D) from tet-acid is desired, substantially shorter contact times areappropriate. Thus, at a cathode potential of -1.3 volts and atemperature of about 25°-28° C., the product mixture may (depending oncathode activity) comprise up to 55 mole % of trichloro acids, about 40mole % of 3,6-D and about 5 mole % of tet-acid, after 2 hours contact.After about 3-3.5 hours contact, approximately equal amounts of the"tri-acids" and 3,6-D, and essentially no tet-acid, will be present.

The electrical requirements for the electrolysis are as follows.

The cathode potential, relative to a standard, saturated calomelreference electrode, should be within the range of from about -0.8 toabout -1.8 volts; this potential preferably is from about -1.2 to about-1.5 volts for 3,6-D preparation and a potential of from about -1.3 toabout -1.4 volts appears to be optimal. For the preparation of 3,4,6-Tand 3,5,6-T (by tet-acid reduction), a potential of from about -0.8 toabout -1.2 volts appears to be better; note Example 11 herein.

(At a cathode potential of -1.3 volts, enough hydrogen is produced (atthe cathode; by electrolysis of water) to account for at least 5% of thecell current. At cathode potentials more negative than about -1.5 volts,hydrogen evolution is pronounced and can account for over 10% of thecell current.)

The current density, in amperes per cm² of projected cathode surface(face nearest to anode) should be within the range of from about 0.005to about 0.085; 0.08 appears to be optimal, i.e., results in a highlevel of tet-acid (or tri-acid) conversion without substantial anodicoxidation of the reduction products thereof.

The cell voltage (potential difference between the anode and cathode) isdetermined (for a given current flow) by the resistance through the celland of course is kept as low as is practical. However, this potentialwill usually be about 2 volts.

Current efficiencies of better than 90% have been attained in thepractice of the present invention on a laboratory scale. On a pilotplant scale, efficiencies of about 70 to 80% have, so far, been moretypical.

If it is elected to employ a porous barrier, such as a diaphragm orporous ceramic cup, between the catholyte and anolyte, the anolyte mayconsist simply of an aqueous base--such as, for example, 10% aq. NaOH.It is highly preferred not to use a barrier, i.e., to immerse both anodeand cathode in a single, undivided, agitated body of a basic aqueoussolution of the tet-acid salt.

It is essential to efficient operation of the electrolysis process thatthe catholyte (and anolyte) be sufficiently agitated, as by magneticstirring, for example. High shear or intense stirring is not necessarybut the degree of agitation should be such that no volume element of thesolution differs in polychloropicolinate or hydroxyl content from theaverage for the solution as a whole by more than a few percent.

Suitable cells for the practice of the invention comprise an activesilver cathode, as defined earlier herein, an anode which preferablyconsists of graphite, a standard reference electrode (such as asaturated calomel electrode in a Luggin capillary) positioned so as tojust touch the cathode, a stirring means such as a magnet bar, means forcollecting (separately) the gases evolved at the cathode and anode,means--such as a potentiostat--for indicating and controlling thecathode and cell voltages, and a pair of electrical leads for connectionto a source of D.C. All portions of the cell which come in contact withthe catholyte/anolyte of course should be resistant to basic, aqueoussalt solutions, or at least should be incapable of providing theretometal ions which will plate out on the cathode. Materials which havebeen found or are considered suitable as the container component of thecell are glass, silver-plated metals, Lucite, graphite and othermaterials commoly employed in chlor-alkali electrolytic cells.

It is generally preferred to use a cylindrical cathode (such as acylindrical, silver, 20 mesh screen, for example) around a central anodewhich may or may not be of conforming shape and is spaced about 1-2.5 cmfrom the cathode. However, the container, if it consists of a conductivematerial, such as graphite, for example, may also function as an anodewhich surrounds the cathode. Similarly, a silver or silver-clad,conductive container can also function as the cathode. (Of course,appropriate safety precautions--such as insulation or grounding--shouldbe taken if the container is conductive.

Preferably, the cell is provided with a polarity reversing means (foractivation or reactivation of the cathode) and a temperature controlmeans, such as a thermostatically-controlled water bath.

Recovery of the gases evolved at the electrodes is readily accomplishedin a conventional manner. Work-up of the reaction mixture(catholyte/anolyte) for 3,6-D recovery is simple and straight forward.The 3,6-D salt is precipitated as the free 3,6-D acid by acidificationto a pH of about 0.5, as with c. aq. HCl, for example, and is thenseparated by filtration or by dissolution in an organic solvent(dichloromethane, for example) which is essentially immiscible withwater. The crude 3,6-D may be recovered in amounts equivalent to 90 to99% of the theoretical yield and in a purity of up to about 98%, byevaporation of the dichloromethane. The 3,4,6-and3,5,6-trichloro-2-picolinic acids, can be removed by recrystallizationof the crude product from an aqueous solvent (such as water, brines oraqueous alcohol, for example) freed of solvent and recycled to thereduction. In those applications where inclusion of the trichloroacidsis not a problem, the crude 3,6-D may be used "as is". It is possible toobtain a crude 3,6-D product that contains very little of thetrichloroacids by prolonging the electrolysis beyond the point at whichthe reaction rate becomes so low that the current efficiency drops offsubstantially.

When the process is operated for production of trichloroacids (and3,6-D), the mixed trichloroacids may be recovered from the 3,6-D motherliquor as a second or third crop. The mixture can then be resolved byknown separatory techniques, such as preparative chromatography. 3,4,6-and 3,5,6-T melt at 128° C. and 144° C., respectively.

The active silver layer on the cathode may be modified (partiallydeactivated) by deliberately occluding a portion (preferably a minorproportion, i.e., less than 50%) of its surface with base metaldeposits. This may be done, for example, by immersing the activatedelectrode in an aqueous base containing base metal cations andcathodically polarizing it.

The following examples are for purposes of illustration and are not tobe construed as limiting the present invention in a manner inconsistentwith the claims appended herewith.

EXAMPLES I. Preparation of Active Silver Cathodes Suitable forEmployment in the Process of the Invention. (Not Illustrative of thatProcess Itself.) Example 1 --Cathodes comprising an active silver layeradhered to a silver foil

(a) A rectangular (0.005"×2"×3") piece of smooth silver foil is immersedin 10% aqueous caustic containing several hundred parts per million ofcolloidal silver oxide (formed upon addition of a dilute silver nitratesolution to the caustic). A counter electrode is also immersed in thecaustic and the silver foil is cathodically polarized at a potential(relative to a saturated calomel electrode) of about -1.5 volts and thepotential difference between it and the anode is adjusted to give acurrent of about 1.5 amperes. Reduction of silver oxide at the cathodesurface is allowed to proceed for about half a minute.

A small portion of the silverized foil is cut off, air-dried andobserved to have a rough matte-white appearance. The remainder of thefoil is kept immersed in the catholyte and is subjected (by means of athrow switch) to a series of four polarity reversals at thirty secondintervals. The resulting "anodized" foil is removed from the cell,air-dried and found to have the appearance of dark brown to black foam.

Both portions of the foil (on the side facing the anode) are examined byscanning electron microscopy (SEM), X-ray dispersive fluorescence andelectron diffraction (reflection). The other layer of the matte-white(unanodized) foil is "seen" to be a deposit of closely spaced, generallyhemi-spherical "bumps" which are contiguous with the underlying foil,have maximum dimensions of up to about 25 microns and are composed ofdensely packed, face-centered, cubic silver microcrystals (about 0.05 to1 micron in "diameter"), the bumps being laterally connected at theirbases and in some instances "fused" together to form polypartite bumpshaving maximum dimensions of from about 30 to about 50 microns.

The microscopic topography of the anodized foil is essentially the same,but the bumps are coated with a relatively thin layer of a silverdeposit having the appearance of a "moss" at 100 magnifications. Themoss is found to consist of loosely packed, face-centered, cubic silvermicrocrystals having a narrow-size distribution around an averagemaximum dimension of about 0.05 microns. At a magnification of 10,000×,the "moss" looks like sponge cake and may be described as having a"spongy" character.

(b) A silver layer is electroplated from an ammoniacal solution ofsilver nitrate on a nickel substrate, peeled off as a foil and sampledfor examination by SEM, X-ray fluorescence and electron diffraction. Thesurface of the foil which has been exposed to the silver solution,washed and air dried, is matte-white in appearance and, at 2000magnifications, is "seen" to have a surface layer of generallyirregular, flattened "bumps" which have maximum dimensions of up toabout 25 microns and are so embossed with flattened, irregularly shaped,micro-protrusions as to have an etched appearance. Some of the bumpsconsist of portions of well defined, individual crystallites havingmaximum dimensions of up to about 6-10 microns. At a magnification of50,000×, all discrete surface areas are relatively smooth in appearance.

The remainder of the foil is anodized in the preceding manner (a,above), washed, air-dried and examined by SEM, etc. It has a brown colorand is found to be coated with a layer of generally dendriticprotrusions, of which the more elongate are joined at their bases intoclusters--in some instances being largely fused together. The individualdendrites are seen (at 2000×) to have "diameters" of about 5-10 micronsand lengths of about 10-20 microns. These dendrites are composed ofclosely packed, face-centered, cubic silver microcrystals and thedendrite surfaces are seen (at 50,000×) to be defined by an outermost,porous layer of discrete but cohered, generally spherical to oblate,silver crystalites varying in size (maximum dimension) from about 0.05to 0.07 microns.

(c) A piece of clean silver foil, essentially the same as that employedin part (a) of this example, is examined by SEM and is "seen" to have asurface resembling (at 500×) that of a paper towel (at 1×). At 10,000×,this surface shows a pattern of grain boundaries resembling incipientcracks between slight elevations in an otherwise flat body of mud.

The foil is immersed in 6% aqueous NaOH in which a counter electrode isalso immersed and is anodized, as follows, for 3 minutes. The potentialof the foil (relative to a saturated calomel electrode) is raised from 0to +0.3 volts in about 30 seconds and then, gradually, to +0.6 voltsduring the following 2.5 minutes. The potential is then graduallyreduced to 0, and finally to -1.3 volts.

The anodized foil, together with enough of the cell liquor to keep itcovered, is then sealed in a glass container and submitted for promptexamination by SEM, X-ray dispersive fluorescence and electrondiffraction.

A sample of the wet, anodized foil is cut off, glued wet to an SEM stuband immediately placed in the SEM column for imaging. Another sample iscut off and alowed to air dry 24 hours before being scanned. Theremainder of the anodized foil is allowed to age 24 hours in the cellliquor and then scanned.

The freshly activated (wet) foil sample is "seen", at 12,500×, to becovered with an adherent, highly porous blanket of cohered, angularparticles consisting predominantly of more than one (face centered,cubic) silver microcrystal each and ranging in size (maximum dimension)from about 120 A (0.012 micron) to about 0.5 micron.

The structure of the blanket on the air-dried sample is found to beuniformly more condensed and to comprise a high proportion of relativelylarge (0.2-0.5 microns maximum dimension) particles readily recognized(at 25,000×) as generally cubical in shape.

The coating on the wet-aged foil sample is found to have condensed lessuniformly, having pulled apart laterally in some places to exposeunderlying, essentially smooth surface areas. The proportion ofgenerally cubical particles is lower and they are less well defined and,on the average, smaller (up to about 0.3 microns, maximum dimension).

EXAMPLE 2--Silver Powder Cathode

An amount of hydrous silver oxide(s) precipitate containing 2 grams ofsilver is formed by adding a dilute (2%) aqueous silver nitrate solutionto a stirred body of 5% aqueous NaOH. The precipitate is filtered outand transferred (wet) to a flat, appropriately cut piece of 100 mesh,stainless steel screen, which is then folded and edge-crimped to form anelectrically conductive, 5 cm×7.5 cm envelope containing--andrestraining--the oxide precipitate. The envelope is then immersed in 5%aqueous NaOH, in which a graphite counter electrode is also immersed,and cathodically polarized to reduce the oxide(s) to powdered silver.The current gradually drops off, over a period of 15 minutes, from aninitial value of about 4 amperes to a then steady value of about 0.5amperes (at an average cathode potential of about -1.5 volts), and thereduction is essentially complete.

A sample of the resulting silver powder is removed, together with someof the cell liquor, and submitted for immediate SEM (etc.) examinationwithout being dried. The powder particles are found to be essentiallythe same as those making up the coating on the freshly activated, wetfoil sample in part (c) of Example 1.

II. Laboratory Scale Reductions of Tetrachloro-2-picolinic Acid(Tet-acid) to 3,6-dichloropicolinic Acid (3,6-D); Cathode Activatedin-situ. Example 3--(Cell A)

To a 300 ml glass beaker, containing a magnetic stirring bar and mountedon a magnetic stirrer, is charged a solution of 15 grams of reagentgrade NaOH pellets in 150 ml of distilled water. A planar, 5 cm×7.5 cm,20 mesh, silver screen cathode, a saturated calomel reference electrodeand a planar, 5 cm×7.5 cm×2 mm graphite anode plate (spaced 1 cm fromthe cathode) are immersed in the upper portion of the solution. Thestirrer is turned on, a source of direct current is connected across thecathode and anode and 3 ml of water containing 60 milligrams of AgNO₃ isadded to the stirring slurry. The potential of the cathode, relative tothe reference electrode, is held at -1.3 volts and the cell potential isset (@˜2 volts) for an initial current of 3 amperes. 10 grams oftet-acid is added over an interval of 30 minutes. Over a total period of4.5 hours (at about 25° C.) the current falls, exponentially, to lessthan 0.3 amperes and the reduction is terminated. 6.8 grams of whitesolids are recovered by acidification (28 ml c. HCl) and extraction ofthe electrolysis solution with CH₂ Cl₂ and evaporation of the extract.By infrared and gas chromatographic analyses, the crude product is foundto have a 3,6-D content of 92.2 wt. %.

EXAMPLE 4

Example 3 is essentially repeated, except that no silver nitrate isadded and the cathode is activated by several polarity reversals of afew seconds each (stirrer off). 7.2 grams of crude product (97.9% oftheory), having a 3,6-D content of 91 wt. %, is recovered.

EXAMPLE 5

Example 4 is essentially repeated, except that when about 95% of theinitially charged tet-acid has been converted, another 10 grams oftet-acid and 3 grams of NaOH are added and the electrolysis is continuedfor a total of 9 hours. 14.6 grams (99.3% of theoretical yield) of crudeproduct having a 3,6-D content of 91.4 wt. % is obtained.

The overall mole ratio of tet-acid to NaOH for this run is 0.17 and thefinal OH⁻ /Cl⁻ ratio (assuming 3,6-D as the only reaction product) isapproximately (18/40-(3×0.914×20/261)÷(2×0.914×20/261)=1.7. The finalwt. % of NaOH in the reaction mixture is about[18-40(3×0.914×20/261)]×100/188=5.1%. (If the weights of H₂ and O₂evolved in the course of the reaction were taken into account, thisfigure would be somewhat higher.)

Example 6

A series of six experimental reductions (a-f) is carried out, using acylindrical, 20-mesh silver screen cathode, which is activated byanodization, i.e., essentially in the manner of Example 4 above. Thecathode is vertically disposed around a planar, 5 cm×7.5 cm×2 mmgraphite anode plate and has a diameter of about 7 cm. The D.C. voltagesource employed is a Model 317 Potentiostat (Princeton Applied Research)and the cell (either a 300 cc or 600 cc glass beaker, depending on thevolume of the reaction mixture) is partially immersed inthermostatically controlled water bath. A saturated calomel referenceelectrode in a Luggin capillary is positioned so that the tip of thecapillary just touches the cathode. Stirring is provided by a magneticstirrer (under the water bath) and a magnetic bar.

The amount of reactants employed, the reaction conditions and durationsand the yields and purities of the crude 3,6-D product obtained (byacidification, extraction and evaporation) are given in Table Ifollowing.

It should be noted that the "final" OH⁻ /Cl⁻ ratios given in the tableare the theoretical minimum ratios calculated by assuming 100%conversion of the tet-acid to 3,6-D.

                                      TABLE I                                     __________________________________________________________________________    REDUCTIONS OF TET-ACID AT SILVER SCREEN CATHODE                                                    Initial                                                                       Mole Final                     Product                   H.sub.2 O                                                                            NaOH   Tet-Acid                                                                             Ratio                                                                              No.   Cathode                                                                            Current    Reac.                                                                             %     3,6-D               Vol.   Wt. Wt.                                                                              Wt. Wt.                                                                              Tet-Acid                                                                           Ratio Pot'l                                                                              Amps   Temp.                                                                             Time                                                                              Theor.                                                                              Content             Run ml Grams                                                                             %  Grams                                                                             %  NaOH OH.sup.- /Cl.sup.-                                                                  Volts                                                                              Start                                                                            End °C.                                                                        Hrs.                                                                              Yield Wt.                 __________________________________________________________________________                                                              %                   6a.sup.(3)                                                                        150                                                                              15  8.57                                                                             10  5.71                                                                             0.102                                                                              3.40  -1.3 2.0                                                                              0.2 30  6   92    92                  b   150                                                                              12  6.98                                                                             10  5.81                                                                             0.128                                                                              2.42  -1.3 2.0                                                                              0.2 30  6   92    92                  c   150                                                                               5  3.03                                                                             10  6.06                                                                             0.307                                                                              0.13  -1.3 2.0                                                                              --  30  Reac. stopped at                                                              50% conversion.sup.(1)        d   300                                                                              18  5.33                                                                             20  5.92                                                                             0.170                                                                              1.44  -1.35                                                                              1.6                                                                              0.3 26  --  99    91                                                  -1.3        50                                e   200                                                                              20  8.33                                                                             20  8.33                                                                             0.153                                                                              1.76  -1.6 3.5                                                                              --  to  11  90    97                                                              55      Reac. stopped                                             -1.6        25                                f   150                                                                              12  6.98                                                                             10  5.81                                                                             0.128                                                                              2.42  to   4.0                                                                              4.0 to  4                                                             -1.8        30      @ 50%                     __________________________________________________________________________                                                        conv..sup.(2)              Notes:                                                                        .sup.(1) CO.sub.3.sup.- and OCl.sup.- formed.                                 .sup.(2) Excessive H.sub.2 evolution (at cathode).                            .sup.(3) Current Efficiency about 90%.                                   

Example 7

A series of three reductions (a-c) of tet-acid is carried outessentially as in Example 6, except that the silver screen employed as acathode is silver plated (prior to being activated) by cathodicpolarization in an ammoniacal AgNO₃ solution.

Three additional reductions (d-f) are carried out in the same mannerexcept that the cathodes employed are formed of: (a) and (b), a silverplated monel screen, and (c), a silver plated nickel screen.

Collection and analysis of the gases evolved at the electrodes duringthese runs shows that the rate of oxygen evolution (at the cathode) isclose to theory for the overall reaction, as represented earlier hereinand is from about 10 to 15 times the rate of hydrogen evolution (at theanode).

These six experiments are summarized in Table II, following.

                                      TABLE II                                    __________________________________________________________________________    REDUCTIONS OF TET-ACID AT SILVER PLATED SILVER SCREEN CATHODE                                      Initial                                                                       Mole Final.sup.(1)              Product                  H.sub.2 O                                                                            NaOH   Tet-Acid                                                                             Ratio                                                                              No.   Cathode                                                                            Current    Reac.                                                                              %    3,6-D               Vol.   Wt. Wt.                                                                              Wt. Wt.                                                                              Tet-Acid                                                                           Ratio Pot'l                                                                              Amps   Temp.                                                                             Time Theor.                                                                             Content             Run ml Grams                                                                             %  Grams                                                                             %  NaOH OH.sup.- /Cl.sup.-                                                                  Volts                                                                              Start                                                                            End °C.                                                                        Hrs. Yield                                                                              Wt.                 __________________________________________________________________________                                                              %                   7a  200                                                                              24  9.84                                                                             20  8.20                                                                             0.128                                                                              2.42  -1.3 3.5                                                                              0.5 30  4    95   98                  b   300                                                                              24  6.98                                                                             20  5.81                                                                             0.128                                                                              2.42  -1.3 2.0                                                                              0.5 23  --   98   99                  c   300                                                                              36  9.84                                                                             30  8.20                                                                             0.128                                                                              2.42  -1.3 2.0                                                                              0.5 23  12   98   98                  d   150                                                                              12  6.98                                                                             10  5.81                                                                             0.128                                                                              2.42  -1.3 1.0                                                                              0.2 27  7    90   99                  e   100                                                                              6   5.40                                                                             5   4.50                                                                             0.128                                                                              2.42  -1.4 2.0                                                                              0.3 23  6    95   95                  f   150                                                                              12  6.98                                                                             10  5.81                                                                             0.128                                                                              2.42  -1.3 -- --  25  7    92   95                  __________________________________________________________________________     Notes:                                                                        The current efficiency in run a is about 85%. For each of runs bf, the        current efficiency is about 80%.                                              .sup.(1) Theoretical minimum ratio for complete conversion of tetacid to      3,6D.                                                                    

Example 8

A series of four runs (a-d) is made at different tet-acid to NaOHratios, using a pre-cleaned and in-situ activated silver screen cathodewhich is periodically reactivated during the reduction.

To 300 ml of distilled water in a 600 ml glass beaker, a preselectedweight of reagent grade NaOH is added with magnetic stirring, thetemperature being controlled by a water bath at a preselected level. Aplanar, 20 mesh, silver screen cathode, 5 cm×7.5 cm, which has beenimmersed in a 1:1 mixture of water and c. aq. HCl for 10 minutes andrinsed with water, is completely immersed in the resulting basesolution. A planar graphite anode of the same dimensions as the cathodeis similarly immersed in the base solution and is spaced about 1 cm fromthe cathode. The cathode is subjected to a potential (relative to asaturated calomel reference electrode) which is initially justdetectably positive and is then raised to about +0.6 volts over a periodof several minutes. The polarity across the cell is then reversed andthe cathode potential set at about -1.3 volts (cell voltage about 2volts). 5 grams of tet-acid is then mascerated with a 20 ml portion ofbase solution (withdrawn from the cell) and the resulting slurryreturned to the cell, the reducting being thereby initiated. The latterprocedure is repeated until (2 hours) all of the tet-acid to be reduced(35 grams) has been introduced to the cell. The cathode is thenreactivated, by a polarity reversal (to +0.6 volts) of about 3 minutesduration. The reduction is continued for a total time of 8 hours, thecathode being reactivated every 2 hours. The cell current increases froman initial level of about 3 amperes to about 5 amperes when the basesolution becomes saturated with the tet-acid sodium salt, and thendeclines to a final level of about 0.3 amperes.

The cell contents are then worked up by acidification, extraction (3×)with CH₂ Cl₂ and evaporation. (It is found that if the amount of CH₂ Cl₂used in the first extraction is not sufficient, an emulsion forms;however, this is readily broken by adding more CH₂ Cl₂.)

Runs a-d are summarized in Table III below.

                                      TABLE III                                   __________________________________________________________________________    EFFECTS OF TET-ACID/NaOH RATIO AND TEMPERATURE                                                Initial                                                                       Mole Final     Product                                        NaOH      Tet-Acid.sup.(1)                                                                    Ratio                                                                              No.       %   3,6-D                                         Wt. Wt.                                                                              Wt.   Tet-Acid                                                                           Ratio.sup.(2)                                                                       Temp.                                                                             Theor.                                                                            Content                                    Run                                                                              Grams                                                                             %  %     NaOH OH.sup.- /Cl.sup.-                                                                  °C.                                                                        Yield                                                                             Wt. %                                      __________________________________________________________________________    8a 33  8.9                                                                              9.5   0.163                                                                              1.58  38-40                                                                             94  97                                         b  22  6.2                                                                              9.8   0.244                                                                              0.55  38-40                                                                             91.5                                                                              94                                         c  17  4.8                                                                              9.9   0.316                                                                              0.085 38-40                                                                             88  87.5                                       d  33  8.9                                                                              9.5   0.163                                                                              1.58  27-30                                                                             93.5                                                                              97                                         __________________________________________________________________________     Notes:                                                                        .sup.(1) Approx. 97% pure. Yields given are corrected accordingly.            .sup.(2) Theoretical minimum OH.sup.- /Cl.sup.- ratio for 100% conversion     of tetacid to 3,6D.                                                      

III. Operation of the Process of the Invention on a Pilot Plant Scale"Standard" cell, conditions and procedure

A rectangular box having external dimensions of 5.125"×13"×48" (13 cm×33cm×122 cm) was assembled from two 1"×13"×48" LUCITE® backing plates, apre-glued 3"×13"×48" frame formed from 1" thick LUCITE®, two 1/16"thick×1" wide, rectangular Neoprene gaskets and forty 3/8"×11/2" or13/4" bolts. To the inner surface of one of the two backing plates a1/16"×107/8"×40" planar silver screen (20 mesh) was fastened by ten,uniformly spaced, silver-plated Monel, 9/16", individually-gasketedbolts passing through the backing plate. Similarly, a 23/4"×107/8"×40"graphite anode was drilled and tapped (10 uniformly spaced holes) andfastened to the inner face of the other backing plate by rhodium-platedtitanium bolts, leaving a 1/4" gap between the anode and cathode.Through top and bottom openings in the cell (box), connections were madefor circulation of liquid in a circuit comprising the cell (up-flow), asmall heat exchanger, a sump (for addition of reactants) and acentrifugal pump. A saturated calomel reference electrode (in a Luggincapillary) was inserted through an additional opening in the cell topand positioned so it just touched the cathode. Electrical power for thecell was provided by a General Electric metallic rectifier (0-10 volts,0-500 amperes) reversibly connected by leads to the protruding portionsof the cathode and anode bolts.

The silver screen used (in most of the runs) had been pre-plated,in-situ, with silver, by filling the cell with a solution of 36 grams ofAgNO₃ in about 5000 cc of 19% NH₄ OH and cathodically polarizing thescreen (potential vs. SCE, -0.05 to -0.13 volts; 16 amps) for 90minutes, draining the cell and rinsing it out four times with distilledwater.

The cell volume was about 4 liters and the system volume was about 23liters, total.

Before each run, the entire system was rinsed with clean water and thecell was cleaned by filling it with (unless otherwise noted) 1:1water/c. HCl--which was allowed to stand for ten minutes--then drained.The system was then re-rinsed and charged with NaOH and water, which wascirculated briefly. With the pump turned on, the cathode voltage wasgradually increased over a period of several minutes from 0 to about+0.6 volts (vs. the reference electrode) and the rectifier potentialadjusted to hold this voltage for a few minutes. The polarity across thecell was then gradually reversed until the cathode potential was about-1.3 volts (total activation time about 10 minutes).

The NaOH used was either reagent grade pellets or was Mercury Cell 50%NaOH (Georgia Pacific Co.) and was dissolved in (or diluted with)purified water in such proportion that the initial NaOH content of thereaction mixture would be as desired (6-7 wt. %, typically).

A quantity of tet-acid (97% minimum assay) was ground with a portion ofcaustic solution withdrawn from the sump and the resulting mixture wasprocessed with a Cowles disperser and returned to the sump. The pump wasturned on, thereby circulating the tet-acid/caustic mixture through thecell (at a rate of about 38 to 95 liters/minute), and initiating thereduction.

The temperature of the reaction mixture was read at a point between thecell top and the heat exchanger and was maintained within a desiredrange by adjusting the flow of cooling water through the heat exchanger.

The weight of tet-acid charged to the reaction ranged from about 0.8 toabout 1.6 times the weight of NaOH charged.

The average cell current ranged from about 98 to about 188 amperes,corresponding to nominal current densities of from about 0.036 to about0.053 amps/cm² (taking the projected surface area of the cathode as107/8"×40"=435 in², or 2806.4 cm²). Since the cathode was a screen, theactual current densities probably ranged from about 0.05 to about 0.07amps/cm². Initial cell currents were as high as about 315 amperes butthe currents just prior to run termination were as low as about 16amperes (depending on the temperature).

The course of each reaction was followed by sampling the cell (system)contents periodically and potentiometrically titrating for Cl⁻ contentwith 0.1 N AgNO₃ solution. When the rate of Cl⁻ content increase becamevery low and the current rate of Cl⁻ had dropped to a base-line value,the reaction was terminated and the system drained.

The composition of the reaction mixture was determined by gas phasechromatography (GPC). A 150 ml aliquot of the reaction mixture (usuallyan essentially homogeneous aqueous solution) was acidified to pH 1 andextracted three times with CH₂ Cl₂. The combined extracts were driedover Na₂ SO₄ and stripped in a rotary evaporator to a pot temperature of45°-50° and the resulting solid (or semi-solid) residium dried in vacuofor 1 hour at 45° C., cooled and weighed. An approximately 0.1 gramsample was weighed out, combined with an equal weight of1,2,3,4-tetrachlorobenzene (as an internal standard) and with 1 ml ofBSA (N,O-bis(trimethylsilyl)acetamide). The resulting mixture was heatedin a REACTI-THERM® reactor (Pierce Chemical Co.) for 10-15 minutes at60° C., to convert the various picolinic acids to the correspondingtrimethylsilyl esters. It was then injected in a GPC apparatusprogrammed for a pre-selected time/temperature profile-starting at 160°C. Detection was by means of thermal conductivity differences and theresponse factors for the several anticipated components of the samplehad been pre-determined with pure standard samples.

Example 9 Preparation of 3,6-D

The essential data for nine runs (a-i) carried out in the precedingmanner are given in Table IV, below. It should be noted that incalculating the % theoretical yields obtained in these runs, it wasassumed that the overall weight loss experienced in the course of thereaction was entirely due to evolution of oxygen (which was generallynot strictly correct). Also, the purity of the tet-acid startingmaterial was taken as 97%, even though some of the tet-acid used assayedas high as 97.9%, because reliable assays were not available for alltet-acid supplied. Thus, apparent yields of 100% (or higher) obtained insome runs have been discounted (in Table IV) to a maximum value of 99%.

In addition to the amount of 3,6-D present in the final product, thecontents of tet-acid, trichloropicolinic acids, monochloro-,4,5-dichloro- and 4-hydroxy-3,5,6-trichloropicolinic acids present werealso determined for some runs. However, only very minor amounts of thelatter several impurities were found and columns are not provided forthese minor components in the Table.

The theoretical minimum OH⁻ /Cl⁻ number ratio calculated for each of theruns in the Table was about 0.6 and the actual final ratios, determinedanalytically, were in good agreement with this value.

                                      TABLE IV                                    __________________________________________________________________________    PILOT PLANT SCALE REDUCTIONS                                                  OF TETRACHLORO-2-PICOLINIC ACID                                                  Total          Initial        Average                                         Grams  Wt. %   Mole Ratio                                                                          Reac.                                                                             Cathode                                                                            Cell Current                                                                             Run                                                                              % Theor.                          H.sub.2 O, NaOH                                                                      Tet-                                                                              Wt. %                                                                             Tet-Acid                                                                            Temp.                                                                             Potential                                                                          Current                                                                            Efficiency                                                                          Time                                                                             Yield                                                                              Wt. %                     Run                                                                              Tet-Acid                                                                             Acid                                                                              NaOH                                                                              to NaOH                                                                             °C.                                                                        Volts                                                                              Amps %     Hrs.                                                                             Solids                                                                             3,6-D                     __________________________________________________________________________    9a.sup.(1)                                                                       22436  5.8 7.0 0.1270                                                                              28-30                                                                             -1.28                                                                               98  79     6.5                                                                             98   95.1                      b.sup.(2)                                                                        22436  5.8 7.0 0.1270                                                                              23-30                                                                             -1.25                                                                              105  71     6.5                                                                             92   98.0                      c  23170  9.8 6.3 0.2385                                                                               8-37                                                                             -1.28                                                                              112  75    11.0                                                                             99   97.9                      d  23170  9.8 6.3 0.2385                                                                              20-35                                                                             -1.28                                                                              111  69    11.5                                                                             97   97.0                      e.sup.(3)                                                                        23170  9.8 6.3 0.2385                                                                              19-35                                                                             -1.28                                                                              119  77    10.0                                                                             99   96.6                      f.sup.(3)                                                                        23170  9.8 6.3 0.2385                                                                              16-35                                                                             -1.30                                                                               84  73    14.5                                                                             99   96.0                      g.sup.(3)                                                                        23170  9.8 6.3 0.2385                                                                              15-35                                                                             -1.29                                                                              104  80    11.0                                                                             99   98.0                      h.sup.(3)                                                                        23170  9.8 6.3 0.2385                                                                              11-35                                                                             -1.29                                                                              135  76     9.0                                                                             99   94.5                      i.sup.(1),(3)                                                                    23170  9.8 6.3 0.2385                                                                              16-36                                                                             -1.29                                                                              151  72     8.3                                                                             99   95.1                      __________________________________________________________________________     Notes:                                                                        .sup.(1) Cathode not cleaned before run.                                      .sup.(2) Cathode (cell) cleaned with 1:9 c. HNO.sub.3 /H.sub.2 O.             .sup.(3) Cathode anodized insitu at 4 hour intervals, during run.        

Example 10 Effect of Metal Impurities in Aq. NaOH Used; SubstantialTrichloro-acid Contents in Product

Three runs (a-c) were carried out essentially as in Example 9, exceptthat the cathode (cell) was not cleaned before any of these runs. TheNaOH solution in each run was made up from 50% NaOH subsequently foundto contain about 20 ppm of base-metals which are detrimental to theefficiency of silver cathodes for the production of 3,6-D. (Anexperimental silver screen cathode used 8 hours in a basic aqueoussolution derived from the contaminated NaOH was found, by X-rayfluorescence, to have 16% of its surface occluded with iron, about 1% bynickel, 2.3% by copper and about 0.7% by lead and (perhaps) zinc.)

The conditions and results (including the content of trichloropicolinicacids in the product) for these runs are given in Table V, below. Again,the contents of by-products (other than the 4-hydroxy derivative oftet-acid) were negligible and are not given.

                                      TABLE V                                     __________________________________________________________________________    PRODUCTION OF MIXTURES OF 3,6-D AND TRICHLORO- PRECURSORS THEREOF,            USING METALS-CONTAMINATED 50% NaOH                                            Total            Initial            Cur-                                      Grams            Mole               rent   %                                  H.sub.2 0,                                                                             Wt. %   Ratio                                                                              Reac.                                                                             Cathode                                                                            Average                                                                            Effi-                                                                             Run                                                                              Theor.                                                                            Weight Percent of                 NaOH  Tet-                                                                              Wt. %                                                                             Tet-acid                                                                           Temp.                                                                             Potential                                                                          Current                                                                            ciency                                                                            Time                                                                             Yield                                                                             Tet-                                                                             Tri-                                                                              3,6-                                                                             4-                   Run                                                                              Tet-acid                                                                            acid                                                                              NaOH                                                                              to NaOH                                                                            °C.                                                                        Volts                                                                              Amps %   Hrs.                                                                             Solids                                                                            acid                                                                             acids.sup.(1)                                                                     D  Hydroxy.sup.(2)      __________________________________________________________________________    10a                                                                              63880 3.4 4.1 0.1271                                                                             18-30                                                                             -1.56                                                                              188  <33  6.5                                                                             --  3.0                                                                              39.3                                                                              45.0                                                                             1.9                   b 24850 1.0 7.2 0.0213                                                                             25-31                                                                             -1.26                                                                               22  <38  7.0                                                                             --  8.6                                                                              23.8                                                                              57.8                                                                             2.7                   c  5160 5.8 7.0 0.1271                                                                             22-26                                                                             -1.27                                                                               12   16 24.0                                                                             78  6.2                                                                              34.6                                                                              49.4                                                                             ND.sup.(3)           __________________________________________________________________________     Notes:                                                                        .sup.(1) About 99% 3,5,6T and 1% 3,4,6T.                                      .sup.(2) 3,5,6Trichloro-4-hydroxy-2-picolinic acid.                           .sup.(3) None detecatable.                                               

IV. Laboratory Scale Preparation of Trichloro Acids Example 11 Effect ofLower Cathode Potential and Higher Temperature on Product Composition

A mixture of 8 grams of 50% NaOH (contaminated with base-metals, as inExample 10), 100 cc water and 7 grams of tet-acid was electrolyzed forabout 6 hours at a temperature of 40° C., using a freshly anodized, newsilver screen cathode and a graphite anode. The cathode potential andcell current after successively longer contact times were as follows:

    ______________________________________                                        Contact        Cathode      Cell                                              Time           Potential    Current                                           Hrs.           Volts        Amps                                              ______________________________________                                        0              -1.00        0.8                                               1.5            -1.00        0.8                                               3.5            -1.00        0.6                                               5.5            -1.00        0.1                                               6.3            -1.00        0.1                                               ______________________________________                                    

The initial mole ratio of tet-acid to NaOH was 0.268 and the theoreticalfinal OH⁻ /Cl⁻ ratio was 0.364.

Work-up of the reaction mixture by acidification, CH₂ Cl₂ extraction andevaporation gave 5.0 grams of solid product (vs. 5.2 grams theory for3,6-D and 6.1 grams theory for 3,4,6-T and/or 3,5,6-T). GPC analysisshowed the product to have the following composition: 3,6-D, 12.2 wt. %;monochloro-acid(s), 0.2%; trichloropicolinic acids, 81.5%; 4-hydroxyderivative of tet-acid, 0.5%; other materials 0.2%.

What is claimed is:
 1. A process of electrolytic reduction in which achlorine substituent in the 4- or 5-position of a polychloropicolinicacid which is tetrachloro-, 3,4,6-trichloro- or3,5,6-trichloro-2-picolinic acid, is replaced with a hydrogen, saidprocess comprising passing a direct, electrical current to a cathodefrom an anode through a stirred, basic aqueous solution of saidpicolinic acid, said cathode having a surface layer of silvermicrocrystals formed by the electrolytic reduction of colloidal,hydrous, silver oxide particles in the presence of an aqueous base. 2.The process of claim 1 in which said cathode has a potential, relativeto a saturated calomel reference electrode, of from about -0.8 to about-1.8 volts.
 3. The process of claim 1 in which said anode is a graphiteanode.
 4. The process of claim 1 in which an undivided body of saidsolution functions as both catholyte and anolyte.
 5. The process ofclaim 1 in which said cathode is a silver screen.
 6. The process ofclaim 2 in which said solution has a temperature within the range offrom about 5° to about 60° C.
 7. The process of claim 6 in which saidsolution has a pH of at least 13 and contains at least 0.08 hydroxylions per chloride ion present therein.
 8. The process of claim 2 inwhich the anode has a positive potential, relative to said cathode, suchthat the density of said current is from about 0.005 to about 0.085amperes per cm² of projected cathode surface.
 9. The process of claim 4in which:a. said solution is saturated with the base salt of saidpicolinic acid, and, together with undissolved particles of the acid,constitutes a slurry, b. initially, all of said acid, as said salt andas said particles, to be charged, and all of the base to be charged, arepresent in the slurry, c. initially, the number of moles of said acidand salt per equivalent of hydroxyl is within the range of from about0.1 to about 0.2, and d. the electrolysis is continued until at least90% of the acid charged to the reaction has been converted to thecorresponding base salt of 3,6-dichloro-2-picolinic acid.
 10. Theprocess of claim 1 in which a portion of the active silver layer on thecathode is occluded by base metals, said picolinic acid istetrachloro-2-picolinic acid, and the reduction is continued until theratio of trichloro-picolinic acids to 3,6-dichloropicolinic acid in thereaction mixture has attained a maximum.
 11. The process of claim 10,carried out at a cathode potential of from about -0.8 to about -1.2volts.
 12. The process of claim 1 in which more of thepolychloropicolinic acid to be reduced, and/or more of the baseemployed, is added to said solution after at least a portion of saidacid(s) originally charged to the reaction has been reduced.
 13. Theprocess of claim 1 in which said solution is formed by dissolving saidpolychloropicolinic acid in aqueous NaOH.
 14. The process of claim 1 inwhich said polychloropicolinic acid is tetrachloro-2-picolinic acid andthe reduction is continued until 3,6-dichloro-2-picolinate anionsconstitute at least 90 mole percent of the chlorinated picolinate anionspresent in the reaction mixture.
 15. The process of claim 1, operatedfor the coproduction of oxygen and polychloropicolinate anions of thestructure ##STR4## wherein one X is H and the other is H or Cl, saidprocess comprising,providing a solution in water of a hydroxyl ionsource-material and a polychloropicolinic acid of the structure ##STR5##wherein both Z and W are Cl, or one is Cl and the other is H, immersinga cathode in a body of said solution and, while agitating said body,passing an electric current therethrough from an anode to the cathode,said body of solution having a temperature within the range of fromabout 5° to about 60° C., a pH of at least 13, and containing at least0.08 hydroxyl ions per chloride ion present therein, said cathodecomprising a shaped, electrical conductor in intimate contact with awater and hydroxyl ion-containing, immobilized, metastable layer ofaggregated silver microcrystals formed by electrolytic reduction ofcolloidal, hydrous, silver oxide particles in the presence of water andhydroxyl ions, the cathode having a potential, relative to a saturatedcalomel reference electrode, of from about -0.8 to about -1.8 volts, andsaid anode having a positive potential, relative to the cathode, suchthat the density of said current is from about 0.005 to about 0.085amperes per cm² of projected cathode surface, thereby forming anions ofsaid polychloropicolinic acid (A) at said cathode and oxygen at saidanode.